Readability fixes in fio_benchmark.py
Readability fixes in fio_benchmark.py

Make fio_benchmark.py more clear in response to review comments.

mateusz-blaszkowski/PerfKitBenchmarker,syed/PerfKitBenchmarker,mateusz-blaszkowski/PerfKitBenchmarker,kivio/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,meteorfox/PerfKitBenchmarker,kivio/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,AdamIsrael/PerfKitBenchmarker,AdamIsrael/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,syed/PerfKitBenchmarker,lleszczu/PerfKitBenchmarker,lleszczu/PerfKitBenchmarker,meteorfox/PerfKitBenchmarker